Abstract: Community dynamics of major insect in spring and summer cotton fields were investigated according to the theory and method of community ecology from June 26 to August 20, 2007. The community structure dynamics was analyzed based on collected data on relative richness, dominance index, dominant concentration index, diversity index and evenness. The results show a rich constitution of insect communities in spring and summer cotton fields. About 48 insect species, belonging to 35 families and 10 orders are observed. In spring cotton field, the insect community comprises 39 species, 30 families and 9 orders while in summer cotton field, the insect community comprises of 43 species, 30 families and 10 orders. The dominant species are Aphis gossypii, Bemisia tabaci and Tetranychus cinnarinus. The dominant species in spring cotton field is T. cinnarinus from the end of June to the beginning of July, reaching the peak at the beginning July. The dominant species after mid. July is A. gossypii, and B. tabaci from the end of July to mid. August in the two cotton fields. Total individual numbers of insect community vary greatly. The total individual numbers of insect community and pest sub-community in spring cotton fields are always higher than in summer cotton fields. Relative richness of pests in spring cotton fields and that of the natural predator in summer cotton fields are larger. Similarity coefficient of parasitical natural predator is highest (0.949 4) and that of phytophagous pest community is lowest (0.439 4). The highest diversity indices of insect community in spring cotton fields and summer cotton fields are 1.895 3 and 2.095 1 respectively. Changes of diversity and evenness are similar in both cotton fields. However, change in aggregation index of dominance is contrary to that of diversity and evenness, which increases with decline in diversity and evenness.